In optical communications, optical components, such as optical elements and glass fibers, which are only isotropic in the first approximation or which are innately anisotropic are believed to be used. For example, the glass fibers employed in photonic networks may exhibit optical anisotropies due to the manufacturing process and design or due to other circumstances, such as temperature and pressure fluctuations, as well as because of the bending of the fiber itself. These, in part, location-dependent anisotropies may also produce an optical birefringence (or double refraction), which can also vary from location to location in the fiber. The birefringence (or double refraction) may result in two orthogonally polarized natural waves of the light propagating at a different phase velocity in one fiber section under consideration. When an optical signal, e.g., an optical pulse having any polarization, is transmitted through the fiber, the optical pulse is believed to become distorted, i.e., spreads during the course of propagation, due to the difference in the velocity of the various polarization components. This spreading of the optical pulses may limit the transmission rate in the communication system.
The reference of “Optical Equalization of Polarization Dispersion”, J. H. Winters et al., Proceedings of the STIE, Jan. 1, 1992, discusses an optical equalizing system, which can be used to reduce the influences of polarization mode dispersion. It is believed that to generate the control signals for the polarization-controlling elements, the optical received signal is received in its entirety in one or a plurality of receivers and is analyzed accordingly. A further transmission of the optical signal is believed to be no longer possible.
The U.S. Pat. No. 5,793,511 discusses an optical receiver having an equalizing circuit which is able to equalize an optical signal distorted by polarization mode dispersion. For this, the received optical signal is converted into two electrical components, of these, the equalizing circuit generating a quality signal for driving a polarization control element implemented in the receiver.
The European Patent Publication No. 0 716 516 discusses a polarization-diversity detection technique for optical signals transmitted over a single-mode fiber. To compensate for distortions caused by polarization mode dispersions in the fiber, a polarization-diversity detection is carried out. For this, it is believed that the optical received signal is initially separated by a polarization beam splitter into a first and second polarization component. A control signal is believed to be subsequently generated to control a polarization-control element as a function of the phase difference between the two polarization components.